Comprehensive characterization and evaluation of deep shales from Wufeng-Longmaxi Formation by LF-NMR technology

Abstract

Marine deep shale gas represents the main target of future shale gas development in China. In order to identify the key parametric characteristics of deep shales, a comprehensive test method and application study was carried out based on LF-NMR technology. The results show that the NMR T 2 spectrum of shales varied considerably in different states. A new NMR porosity and water saturation test method based on three-time measurements was established. The relative error between corrected NMR and water immersion porosity is reduced from 49.97% to 6.66%, while that between corrected NMR and weighing water saturation is reduced from 32.15% to 6.85%. By testing key parameters such as free water quantity, adsorbed water quantity, and average adsorption thickness, the NMR surface relaxivity of shale is determined and used to transform NMR T 2 spectrum into pore size distribution. The NMR T 2 spectra of individual sublayers also differ significantly. In order to quantitatively describe the distribution of spectral peaks, a new reservoir evaluation parameter, T 2 geometric mean value ( T 2g ), is proposed. Larger T 2g values signify larger pores, broader average pore size distribution, and higher fluid mobility. With this parameter, qualitative description of pore size distribution can be transformed into quantitative characterization. When evaluating shale gas reservoirs, it is important to use multiple key parameters, among which NMR test results are an important indicator. LF-NMR testing is uniquely superior over other test methods as one or more NMR tests can provide abundant parametric information. The presented comprehensive NMR test method offers useful theoretical references for furthering the development and application of this technology in shale gas research. • A three-time NMR test method was proposed for all the basic parameters determination. • NMR surface relaxivity was determined and negatively correlated with TOC. • Parameter T 2g was proposed to quantitatively describe the spectrum peak location and pore size. • One or more NMR tests can provide abundant parametric information for shale evaluation.